1. Field of the Invention
The present invention relates to a carburetor and, more particularly, an improvement of the carburetor with regard to its fuel injection performance in a transient region between the slow operation wherein fuel is principally supplied from a slow fuel supply system and the normal operation wherein fuel is principally supplied from a main fuel supply system.
2. Description of the Prior Art
When a carburetor for an automobile internal combustion engine is operating in the slow speed or idling condition wherein the throttle valve is substantially or completely closed, no substantial vacuum is generated in the Venturi portion and, accordingly, a substantial small amount of fuel is drawn out from the main nozzle. Because of this, the carburetor is generally provided with a slow port which opens to the intake passage at a position close to an end of the throttle valve in its closed position and an idle port which opens to the intake passage and displaced a small distance from the slow port toward the downstream side so that in the slow speed or idling condition fuel is drawn into the intake passage from these ports. The position and the opening of the slow and idle ports are determined in a manner such that when the engine is operating in a predetermined slow speed or idling condition, a predetermined amount of fuel necessary for stable slow speed engine operation or idling is drawn from these ports by a vacuum due to an air flow through a clearance formed between the slow port and the end portion of the throttle valve positioned to closely oppose the port or an intake vacuum applied to the idle port when the throttle valve is completely closed. The flow of fuel through a slow fuel supply system including the slow port is controlled by a jet means called "slow jet". In the prior art carburetors, a slow fuel passage including the slow jet is branched from a main fuel passage extending from a float chamber to the main nozzle at a portion downstream of a main jet incorporated in the main fuel passage. This is due to the reason that if the main and slow fuel passages are individually connected to the float chamber, when the throttle valve is gradually opened from its substantially closed position, the supply of fuel from the slow fuel supply system is maintained for a while even when the supply of fuel from the main fuel supply system has started, thereby causing an overrich fuel-air mixture in a transient region from a slow to a medium speed operation. When the slow fuel passage is branched from the main fuel passage, at a downstream portion of the main jet, the amount of fuel supplied from the slow port is instantly reduced as the supply of fuel from the main nozzle begins, thereby ensuring transition of fuel supply from the slow fuel supply system to the main fuel supply system as the throttle valve is opened. In this case, therefore, the aforementioned overrich fuel-air mixture in the transient region is avoided.
FIG. 1 shows the fuel flow performance of conventional prior art carburetors. In FIG. 1, the abscissa represents the opening of the throttle valve while the ordinate represents the flow of fuel supplied from the carburetor to the intake passage. In the figure, curve G.sub.I shows the flow of fuel supplied from the idle port, curve G.sub.L shows the flow of fuel supplied from the slow port, curve G.sub.M shows the flow of fuel supplied from the main nozzle, and curve G.sub.T shows the total of the three fuel flows.
In this conventional prior art carburetor, when the main fuel supply system begins to supply fuel in the transient region from the slow speed condition to the normal operating condition (the region shown by C in FIG. 1), the fuel supply from the slow port is instantly reduced as explained above and, therefore, there is no problem of causing an overrich condition of fuel-air mixture in the transient region. On the contrary, however, since there is a delay in the actual delivery of fuel from the main nozzle, a temporary shortage of fuel is caused thereby causing a supply of an overlean fuel-air mixture in the transient region which inhibits the performance of the engine in acceleration from the idling or slow speed condition. Particularly when the engine is operated by a relatively lean fuel-air mixture for the purpose of purifying exhaust gases, the engine may readily stall due to the aforementioned temporary shortage of the fuel supply. Therefore, this situation is an important problem to be solved in the lean combustion engine.